1. Field of the Invention
This invention relates generally to combiners/decombiners, both of the selective wavelength type such as optical multiplexers/demultiplexers, for example, arrayed waveguide gratings (AWGs) or elliptical supergratings (ESGs) and of the non-selective wavelength type such as free space region couplers, star couplers or multi-mode interference (MMI) couplers. More particularly, this invention relates to the deployment of what we refer to as “tilted” optical combiners/decombiners of both the wavelength selective and the non-selective type.
2. General Definitions
Reference is made in this disclosure to a “optical combiner/decombiner” when only the application of the optical combiner itself is generally discussed. However, as it will be evident to those skilled in the art, the principals of such optical combiners shown in this disclosure can be equally be utilized as optical decombiners such as illustrated in an optical receiver photonic integrated circuit (RxPIC) disclosed in U.S. patent application Ser. No. 10/267,304, filed Oct. 8, 2002 and published on Feb. 19, 2004 as Pub. No. 2004/0033004, which application is incorporated herein by its reference.
Reference in this description to signal channels is nominally either a modulated semiconductor laser source or a semiconductor laser source and its associated modulator, each referred to as a “modulated source” and providing a modulated signal output. In this context, such a signal channel is also an optical waveguide with one or more accompanying integrated elements, such as, but not limited to, a laser source, a modulator, a photodetector or a semiconductor optical amplifier (SOA), a variable optical attenuator (VOA) or a combination VOA/SOA. Thus, a train of elements formed in each signal channel comprising a modulated source and other accompanying integrated elements for a signal channel array comprising two or more such channels integrated in a photonic integrated circuit (PIC).
“Laser emission wavelength” means emission output wavelength of a laser source in a particular signal channel formed in a photonic integrated circuit (PIC) such as an optical transmitter photonic integrated circuit or TxPIC.
“Active region wavelength” means the wavelength of the photoluminescence (PL) peak or the gain peak of an active region wavelength spectrum, for example, of laser source and/or modulator formed in a photonic integrated circuit (PIC) signal channel where the channel laser source and modulator share the same active region and active region bandgap and/or share the same active region and active region bandgap with adjacent, integrated signal channels. For purposes of brevity in this invention, PL peak and gain peak are used interchangeably although they are slightly different as is known in the art.
The terms, “laser(s)” and “laser source(s)”, are used interchangeably in this disclosure.
An “active region” as employed in the description of this disclosure means the region in a semiconductor device where carrier recombination and/or light absorption occurs which may be comprised of a single semiconductor active layer or multiple semiconductor layers with any associated optical confinement layers, as is well known to those skilled in the art. An active layer, for example, may be a single high refractive index active layer or may be multiple active layers such as in the case of multiple quantum well layers and barrier layers which are, together, commonly referred to as an active region.
Also, examples of the inventive optical combiners/decombiners illustrated in this disclosure are referred to as “tilted” in the sense that the inputs or the outputs of the combiner free space region or regions are spaced from or offset relative to a longitudinal centerline of the free space region, but still remain within a zero order band of the combiner free space region, rather than being positioned on and about that centerline of a zero order band of an input free space region of the combiner, which is the common practice in such devices for optimized combiner passband results. In terms of the foregoing description, “offset” of the inputs or outputs, as the case may be, means that they are spatially to one side of the longitudinal centerline of the free space region, i.e., they are spatially offset.
3. Description of the Related Art
Reference is made to FIG. 1 which shows a transmitter photonic integrated circuit or TxPIC 10 which has been disclosed in U.S. patent application Ser. No. 10/267,331, filed Oct. 8, 2002, and published on May 22, 2003 as Pub. No. US 2003/0095737 A1, as well as disclosed in U.S. patent application Ser. No. 11/268,325, filed Nov. 7, 2005, both of which applications are owned by a common assignee herein and are incorporated herein by their reference. As shown in FIG. 1, monolithic PIC chip 10 comprises groups of integrated and optically coupled active and passive components formed in a series of signal channels, identified as channel Nos. 1 through 12 in FIG. 1. It should be noted that any number, N, of channels can be formed in chip 10, such as less than twelve channels or greater than twelve channels. Each signal channel includes a modulated source 13 comprising a laser diode 12, such as a DFB semiconductor laser or a DBR semiconductor laser, and an electro-optic modulator 14. Each laser source 12 operates at a different wavelength, λ1-λN where N here is equal to twelve, where the group of wavelengths provides a wavelength grid that may be commensurate with a standardized wavelength grid, such as the ITU wavelength grid, but may also operate on a non-standard or form a wavelength grid with nonuniform wavelength spacing. On the other hand, the wavelength grid need not be any particular standard. Laser diodes 12 are respectively provided with an associated electro-optic modulator 14 as shown in the example here. Thus, the CW outputs of laser diode sources 12 are shown optically coupled to respective modulators 14. Modulators 14 may be electro-absorption modulators (EAMs) or Mach-Zehnder modulators (MZMs) as detailed in patent application Ser. No. 10/267,331, supra. It is within the scope of this disclosure that rather deploying electro-optic modulators 14, laser diode sources 12, themselves, may be directly modulated. Modulators 14 each apply an electrical modulated signal to the CW light from laser diodes 12 producing an optical modulated signal for transmission on an optical link of an optical transmission network. The modulated source outputs from modulators 14 may be optically coupled to a photodetector 16 for the purposes of monitoring the output power or signal characteristics received from modulators 14. The on-chip deployment of photodetectors 16 is optional. Photodetectors 16 may also be fabricated off-axis of a channel or in-tandem with the optical train elements 12, 14 and 15. Photodetectors 16 may be PIN photodiodes, MSM photodetectors, or avalanche photodiodes (APDs). Also, each signal channel may include an electro-optical amplitude varying element (AVE) 15. An AVE channel element 15 may be a variable optical attenuator (VOA), a semiconductor optical amplifier (SOA), a gain-clamped SOA (GC-SOA), or a combination VOA/SOA, the latter of which is also referred to in the second of the above identified and incorporated patent applications as a “ZOA”. A ZOA is capable of biasing either positive or negative to adjust the optical power level in a channel to be higher or lower, respectively. The train of these elements 12, 14, 15 and 16 are numerically identified for only signal channel No. 1 in FIG. 1 so that it should be understood in this description that they are all the same for the remaining signal channel Nos. 2 through 12.
An approach in the operation of TxPIC 10 is to operate all semiconductor lasers 12 at a predetermined power level throughout the life of the PIC chip 10. In this way, the need for control logic to vary laser current to control power level changes over life is eliminated, which current changes can also change the effective emission wavelength of the laser which must be maintained on a desired wavelength grid in optical transmission systems. Over the life of the semiconductor lasers 12, the power levels will fall naturally due to certain deleterious aging effects so that the on-chip AVE channel elements 15 are deployed to maintain the predetermined power level such as by boosting the power of the modulated signal. The semiconductor laser deleterious aging effects are the development of leakage current paths in the laser and dopants in the laser semiconductor layers will migrate over time reducing its slope efficiency as well as, to some extent, misalignment of the TxPIC chip output to its package output fiber due to constant temperature variations imposed upon the chip package over time.
As indicated above and as explained in more detail in patent application Ser. No. 10/267,331, supra, modulators 14 may be fabricated as electro-absorption modulators (EAMs), Mach-Zehnder modulators (MZMs) or band edge Mach-Zehnder modulators. The modulated optical signal outputs of modulators 14, via photodetectors 16, are respectively coupled, via waveguides 18(1) . . . 18(12), to an on-chip or integrated wavelength selective combiner, shown here as an arrayed waveguide grating or AWG 20. Waveguides 18(1) . . . 18(12) receive the modulated channel signals from the N channels and provide them as an input to AWG 20. Combiner or multiplexer 20 may also be substituted by another type of wavelength-selective combiner/decombiner, such as an elliptical supergrating, an Echelle grating, a cascaded Mach-Zehnder interferometers (CMZIs), broadband multiplexers of the type shown, for example, in U.S. Pat. No. 6,580,844 (which is also incorporated herein by its reference), a so-called free-space diffraction grating (FSDG) or a quasi-selective wavelength star coupler having a multimode coupling region comprised of waveguides as disclosed in U.S. patent application, publication No. 2003/0012510 (which is also incorporated herein by its reference). Such wavelength-selective combiners or multiplexers are more conducive to high channel signal count on a TxPIC chip 10. However, it is within the scope of this disclosure to practice the invention in connection with non-wavelength selective couplers, such as power couplers, star couplers or MMI couplers which can be employed in particular circumstances. Each of the modulated sources 13 is, therefore, representative of an optical signal channel Nos. 1 through 12 on TxPIC chip 10. There may be, for example, as many as forty (40) signal channels or more formed on a single TxPIC 10.
Each signal channel is typically assigned a minimum channel bandwidth spacing to avoid crosstalk with adjacent optical channels. Currently, for example, 50 GHz, 100 GHz, 200 GHz, or 400 GHz are common channel spacings between signal channels. The physical channel spacing or center-to-center spacing 28 of the signal channels may be 100 μm to 1,000 μm or more to minimize electrical or thermal cross-talk at data rates, for example, of 10 Gbit per second or greater, and facilitate routing of interconnections between bondpad groups 27 for the multiple PIC optical elements 12, 14, 15 and 16. Although not shown for the sake of simplicity, bonding pads may be provided in the interior of PIC chip 10 to accommodate wire bonding to particular on-chip interior electro-optic elements in addition to bond pad groups 27 comprising chip edge-formed bonding pads.
As indicated previously, the respective modulated outputs from electro-optic modulators 16 are coupled into optical waveguides 18(1) to 18(12) to the input of AWG 20 as shown in FIG. 1. AWG 20 comprises an input free space region 19 coupled to a plurality of diffraction grating waveguides 21 which are coupled to an output free space region 22. The multiplexed optical signal output from AWG 20 is shown as provided with a plurality of output waveguides 23 which comprise output venires, or what might be referred to as a plurality of spare outputs that provide optimum performance of the multiplexer to the same set of multiplexer inputs, which outputs are along the zero order Brillouin zone at output face 22A of output free space region 22 of AWG 20. However, the deployment of spare outputs 23 is optional and the output may be to a single output waveguide. Spare output waveguides 23 extend to output facet 29 of TxPIC chip 10 where a selected output may be optically coupled to an output fiber (not shown). The outputs may also be disposed at a small angle relative to a line normal to the plane of output facet 29 to prevent internal reflections from facet 29 back into vernier outputs 23 that may affect stabilized laser wavelength operation. The deployment of multiple vernier or spare outputs 23 provides a means by which the best or optimum output from AWG 20 can be selected having the best overall passband response of AWG 20 with the established wavelength grid of the group of channel signal outputs from the array of laser sources 12. Seven outputs 23 are shown in FIG. 1. It should be realized that any number of such vernier outputs may be utilized beginning with the provision of two of such outputs. Also, the number of such outputs may be an odd or even number.
In operation, AWG 20 receives N optical signals, λ1-λN, from coupled input waveguides 18 which propagate through input free space region 19 where the wavelengths are distributed into the diffraction grating waveguides 21. The diffraction grating waveguides 21 are a plurality of grating arms of different lengths, ΔL, relative to adjacent waveguides, so that a predetermined phase difference is established in waveguides 21 according to the wavelengths λ1-λN. Due to the predetermined phase difference among the wavelengths in grating arms 21, the focusing position of each of the signals in grating arms 21 in output free space region 22 are substantially the same so that the respective signal wavelengths, λ1-λN, are focused predominately at the center portion or the zero order Brillouin zone of output face 22A. Verniers 23 receive various passband representations of the multiplexed signal output from AWG 20. Higher order Brillouin zones along output face 22A receive repeated passband representations of the multiplexed signal output at lower intensities. The focus of the grating arm outputs to the zero order Brillouin zone may not be uniform along face 22A due to inaccuracies inherent in fabrication techniques employed in the manufacture of TxPIC chip 10. However, with multiple output verniers 23, an output vernier can be selected having the best or optimum combined WDM signal output in terms of power and responsivity.
Power output across an array of lasers, when forward biased at identical currents may vary as a function of position across the array for a number of reasons. One possibility is systematic variation from the design specifications to fabrication tolerance. Another is the predictable variation one gets from the application of selective area growth in a fabrication step, for example. Regardless of the cause of the output power variation across the array, power equalization at the chip output is very desirable so that a desired signal to noise ratio (SNR) will be achieved at an optical receiver in an optical transmission network. One approach to the signal output power equalization is to adjust the bias current of each of the lasers to follow the power spectrum curve such that more bias current is provided to the weaker lasers and less bias current is provided to the stronger lasers. However, this not a successful approach because the weaker lasers often have higher current thresholds compared to the stronger lasers. As a result, the weaker lasers will have less power output. To increase the bias current on the weaker lasers to provide improved power equalization across the signal channel outputs means that some lasers will be operating at higher current levels relative to the respective laser current thresholds (ITH) than other lasers so that the overall life expectancy of the TxPIC may be shorter than what is actually possible.
Another approach is to operate all of the lasers at their highest rated power level and then employ in each signal channel an amplitude varying element (AVE), particularly a loss element such as a variable optical attenuator (VOA), to reduce the highest rating power level across the laser array output to a substantially equalized level. As the laser sources age over time, their power intensity reduces so that the negative bias applied to the VOAs, relative to each element, can be correspondingly reduced to maintain the same and original power output level across the laser array over the life of the TxPIC. This approach includes, alternatively, the utilization of ZOAs (or in addition to the VOAs, SOAs) in each signal channel where the power in some of the channels reduced while the power in some of the other channels is increased to achieve power equalization across the signal channels. The downside to this approach is the throwing away of laser power in addition to providing a channel array of biased elements in the circuit that increase both the power budget and thermal budget of the TxPIC chip.
In ordinary combiners/decombiners known in the art, particularly of the wavelength selective type, the output waveguides are coupled to the center of the zero order band about the centerline of the free space region. In this case, all of the inputs and/or outputs to and from the free space region would be substantially parallel with the free space region longitudinal centerline. In this connection, reference is made to FIG. 2 which illustrates how the waveguide inputs of the twelve (12) signal channels are provided to the input free space region 19 of arrayed waveguide grating (AWG) 20 along the free space input edge 33. The inputs, which shown here number thirty-one (31) are formed into the zero order band 34 providing input channels 30 for the purposes of this description which will become clearer later on. Within the input signal channels 30 are the twelve input signal channels 32 from modulated sources 13 shown in FIG. 1, where the channel inputs 18 are centered about the centerline 38 of the zero order band 34 of free space region 19. Also, shown in FIG. 2 is one approximate edge 36 of zero order band 34. It is standard practice to center the waveguide inputs 32 at the center of the zero order band 34 of input free space region 19 of AWG 20. This is because the optimum power in the passband of AWG 20 is generally centered on the centerline 38 of the center or zero order band 34 of input space region 19 as illustrated in FIG. 3. FIG. 3 is a graphic illustration of the power output passband envelope of AWG comprising passband curve or envelope 40. Each dot on curve 40 represents an input channel 30 along the passband envelope or curve 40. The twelve centered channel inputs 18 in TxPIC 10, designated by rectangle 42 in FIG. 3, are the twelve channels approximate to or abounding either side of the peak of normal power distribution through free space region 19 of AWG 20, i.e., the region in curve 40 where its passband envelope is optimum and the most uniform in terms of flatness as compared to other regions of the same curve 40.
We have discovered a new approach that at least reduces, if not eliminates in certain cases, the necessity of an increase in power and thermal budget brought about by the inclusion of at least one AVE in each signal channel. This new approach involves the passband spectrum of the optical combiner integrated in the TxPIC or any other photonic integrated circuit for that matter, such as, for example, the design of arrayed waveguide grating (AWG) 20 and other such wavelength selective combiners/decombiners mentioned later in this disclosure and, more particularly, the particular placement of the input waveguides or channels 18(1) . . . 18(N) relative to the zero order band 34 of input free space region 19 of AWG 20.